This invention relates to a method and apparatus for reading bar code magnetic ink characters which employ a combination of inked bars which are separated by non-inked intervals of varying widths as is done in CMC7 coding, for example.
With coding schemes like the one mentioned, correct reading of a character depends upon accurately detecting and classifying the "white" intervals or spaces between the inked-bars as being "long" or "short" intervals. For example, in CMC7 coding, each character is made up of a combination of seven, vertically-positioned or parallel inked bars of constant width and six non-inked intervals (white spaces) of varying widths located between the inked bars. A character always starts and ends with a black or inked bar, and the character is designed to be human readable in addition to being machine readable.
Some of the factors which cause a reject or misread during machine reading of the characters relate to poor printing of the characters (like broken or "void" bars, or ink spatter), electronically noisy environments (like an electric motor), or variations in the speed at which the characters to be read are moved past an associated read head.
One of the ways in which CMC7 characters are read, for example, is to measure the distance between the trailing edge of one inked bar and the trailing edge of the next inked bar. Because the inked bars are designed to have a constant width, the spacing or interval is obtained by "subtracting", for example, the width of an inked bar from the distance measured. The resulting distance is a measure of the white space or "interval". This interval is then determined to be "long" or "short". The interval could also be considered to include the width of one bar. By examining the pattern of inked bar, long or short interval, inked bar, long or short interval, etc., a character is recognized. For the CMC7 coding mentioned, for example, there are seven inked bars, two wide or long intervals, and four narrow or short intervals used in a predetermined combination for each character.
One of the problems with the prior art method of reading characters described in the previous paragraph relates to noise spikes which occur in reading transition areas. A positive transition area occurs in going from a white interval to an inked bar, for example, while a negative transition area occurs in going from an inked bar to a white interval. The problem mentioned occurs when a noise spike is construed as a negative going signal, thereby distorting the interval between two adjacent inked bars. Such a noise spike might be interpreted as making the interval between two successive bars longer than intended, thereby causing a reject or a misread to occur.